Experiments in marine biochemistry. Homarine metabolism in Penaeus duorarum.

A fractionation procedure has been developed which permits the isolation of 1 to 2 mg of homarine from a single shrimp. This procedure was used to show that homarine is endogenously synthesized by Penaeus duorarum in the free unbound form, and to study the metabolic precursors involved. Injected DL-14Ctryptophan was not converted to 14Chomarine. However, 6-14Cquinolinic acid, a known catabolite of tryptophan, is an effective precursor. 2-14CAcetate and U-14Cglycerol are effectively converted to 14Chomarine while 14Cbicarbonate is poorly utilized. The injection of L-U-14Caspartate resulted in labeled homarine, but the quantity converted was less than expected. Since 14Cglycerol is an effective precursor there is a possibility that quinolinic acid may be formed in P. duorarum by a condensation similar to that of glyceraldehyde 3-phosphate with aspartic acid or a closely related metabolite. It is suggested that decarboxylation of quinolinic acid gives rise to picolinic acid which is methylated to yield homarine. L-methyl-14CMethionine efficiently provides the N-methyl carbon presumably via S-adenosylmethionine.